When using chargeable electrochemical energy storage batteries, for example, as starter batteries for a motor vehicle, it is desirable and in safety-critical fields of operation necessary to be able to identify the state of the energy storage battery.
One problem that occurs in this case, however, is the complexity of the processes in the rechargeable battery, which can be described only with difficulty using scientific methods. For example, DE 195 40 827 C2 discloses an empirical method for determining the state of aging of a battery, in which a battery-specific family of characteristics is predetermined for battery aging. A battery aging value is determined by detecting instantaneous values of the variables which influence battery aging in the battery being monitored, with the aid of the family of characteristics.
Furthermore, DE 199 60 761 C1 discloses a method for monitoring the residual charge and the capability of an energy storage battery to supply power, in which measurement points are determined by means of a number of current and voltage measurements with the energy storage battery in different load states, and in which intersections with a limit voltage level and limit current level are determined by means of straight interpolation lines.
One disadvantage is that these methods need to be matched to the respective physical form of the energy storage battery and to the required function of the energy storage battery by means of constants and functions which have to be determined from a large number of experiments. This applies particularly to batteries in motor vehicles with an internal combustion engine, in which the energy storage battery is subject to completely random influences in the electrical power supply system, in terms of the requirements and operating conditions to which it is subject. For example, the changes in the charging and discharge periods as well as their intensity in conjunction, for example, with the respective battery temperatures are distributed absolutely randomly and are unpredictable, owing to the random nature of the driving cycle.
A further problem is that power reserves in the energy storage battery must be ensured in order to supply safety-relevant loads, such as electrical steering and/or electrical brakes.
A method for measuring the state of health of an energy storage battery when subjected to an electrical load is disclosed in EP 1 116 958 A2. In this case, the energy storage battery has a load profile applied to it, that is to say a defined current or a power is drawn for a fixed time, and the voltage response of the energy storage battery is measured. The lowest and highest voltage values while the load profile is being applied to the energy storage battery is a measure of the state of health (SOH). As an alternative to applying a load profile to the energy storage battery, the state of health (SOH) can also be determined by computation from the state of charge SOC, the dynamic internal resistance Ri and the temperature of the energy storage battery under consideration. The characteristic value for the state of health SOH describes, however, only one measure of the capability of the energy storage battery to supply an amount of energy at the required voltage level.
Furthermore, DE 199 52 693 A1 discloses a method for determining, indicating and/or reading the state of an energy storage battery, in which the battery voltage, battery temperature, charging current, discharge current and/or the no-load current are detected and a controlled variable for the generator that is used for supplying the energy storage battery is derived via the characteristic values for the state of charge SOC as a function of a no-load voltage and the battery temperature, and of the characteristic value for the state of health SOH.
The characteristic value for the state of charge SOC in this case describes the state of charge of the energy storage battery taking into account the amount of charge with which it has been charged or which has been discharged from it.